This invention relates to an article, such as a machine part, having a cast metal portion and at least one sintered metallic portion which bonds closely to the cast metal portion, and to a method of producing the same. For example, this invention is embodied in a rocker arm for an internal combustion engine.
There are many kinds of machine parts which are produced fundamentally by forging or casting of a metal and are required to possess a special property such as high resistance to abrasion only in a local portion having a functional surface. With respect to automotive internal combustion engines, examples of such machine parts are the cylinders each receiving a piston therein, cam shafts having cam-follower portions and rocker arms each having a tip portion which makes rubbing contact with a cam. The inside portion of each cylinder, cam-follower portions of each cam shaft and the tip portion of each rocker arm must have excellent wear-resistant and anti-seizing properties.
In many cases a machine part of the above described category is formed entirely as a single body and then the special property portion is produced as a relatively thin surface layer by plating with a hard metal or by a surface hardening treatment such as carburizing, nitriding or quenching. However, it is not always possible to obtain a sufficiently wear-resistant surface by this method, and it is difficult to produce a desirably thick wear-resistant layer by this method. Also it is known to produce a functionally similar machine part by first forming the special property portion as a separate member by a suitable method such as a powder metallurgical technique and then forming the body portion by a suitable casting method such as die casting with the separately formed portion set in the mould as an insert. However, in the composite machine part obtained by this method the bond at the interface between the body portion and the special purpose portion is not very high because the materials of the two portions do not fuse into a blend. Therefore, it is not rare that the special property portion peels off from the cast body portion during practical use of the machine part under severe conditions.